Materials are the building bricks of everything that we see around us. Where would smartphones, cars, buildings or solar cells be without the right materials? Materials are so self-evident that we almost forget how special they are.
Materials science provides an answer to the questions ‘Why is that peculiar material suitable for that application?’ and ‘Which knobs do we have to turn to get better materials – if the current ones do not satisfy?’
Materials science and materials technology are closely related. In Dutch, these terms are often used interchangeably, but in the English language there is a clear distinction between the two. In the English case, ‘science‘ answers the ‘why’ question, whereas ‘technology‘ answers the ‘how’ question.
On the one hand you would like to know ‘Why is that peculiar material suitable for that application?’ So to get an answer to questions like ‘why are bricks suitable to build houses with’ or ‘why is glass transparent so you can make windows and glass fibres for data transport out of it’?
On the other hand, sometimes it happens that a certain material has already been used for a certain application, but that it no longer fulfils – either or not based on progressive insight. Suppose you could reduce the amount of metal in a car or an aircraft, then you would consume less fuel due to the lower weight of the vehicle that has to be carried along. Consider a different design that uses less material – for example hollow tubes instead of dense bars. But, as an alternative, you can make the metal itself stronger, so you will need less material to obtain the same strength – resulting in a lower weight. In brief: you would like to know which knobs to turn in order to get better materials – which is the field of materials technology.
The essentials of materials science
Here we enter the essentials of materials science: the manufacturing – structure – properties – application chain, and the relations between them (see figure 1). Materials science as a discipline area tells us which influence microstructure – so the structure on the smallest scale – has on the ultimate properties of a material, and therefore on its applications. By turning the knobs on a small scale during manufacturing, it will be possible to establish or improve the properties of a material – and of the product.
Figure 1: The essentials of materials science
If you want to choose the most suitable material for a particular application, then go through the chain from right to left. Start with the requirements that are set by the application: should the material be light, or strong, or flexible, or corrosion-resistant, should it conduct electrical current or not, what is the maximum service temperature, just to mention a few. Preferably with the most important requirement first. After that, make sure that these requirements match material properties, with the desired material as a result. If the result is not completely satisfactory, then you have to adapt the manufacturing process.
Structures of materials
Let’s have a closer look at a few parts of the manufacturing – structure – properties – application chain. Literally, by zooming in to an atomic or molecular scale (see figure 2). This results in the following microstructures:
Crystalline material: virtually perfect, regular, repeating arrangement of atoms in three directions. As a matter of fact, it is one large crystal. Diamond is a good example, as well as sapphire that is used in lenses for cameras in smartphones, or silicon as the basis for computer chips.
Polycrystalline material: a large number of grains, each of them is crystalline. Besides ‘real’ material, grain boundaries are present here. Usually, ceramics and metals are polycrystalline.
Amorphous material: no regular arrangement of atoms; the atoms are intermingled. This microstructure is very well comparable to that of a liquid where the particles are frozen in place. Glass is a good example, as well as some polymers.
Figure 2: Microstructure of materials
Classifying these ‘micro’structures into groups results in three main groups: polymers, metals and ceramics. Polymers are especially popular because you can shape products in one step – that’s why they are also known as ‘plastics’, as an expression of their plastic processing. Rubber is also considered a member of the polymer family – usually a very elastic polymer – be it that natural rubber is originally a natural and not an artificial material.
Also metals owe their popularity to the relative simplicity with which you can shape them into products. From a mechanical point of view, metals are ideal materials: stiff and tough.
Ceramics are very stable materials – usually strong up to high temperatures, and popular due to the abundant occurrence in nature of their raw materials, with clay as the most important raw material for traditional ceramics such as bricks, tiles, sanitary ceramics and tableware. Glass is the transparent ‘brother’ of ceramics: hard as well, electrically isolating and well resistant to chemical degradation.
In addition to these three main groups, composites do exist: combinations of two or more materials, the ‘best of both worlds’, hoping that the properties of the constituents strengthen each other. Reinforced concrete is a composite material where concrete relieves pressure loads and the metal reinforcement (steel bars) handles tensile loads. In glass fibre reinforced polymers, low weight and high strength are combined in one material. Inspiration for composites originates from nature, with wood and bone as examples.
The manufacturing process consists of various steps, and depends on the material. When manufacturing a product ‘from scratch’ out of raw materials, you will have to include a number of manufacturing steps. At first, you have to mine the raw materials and convert them into a material that has to be processed in one or more steps into the right shape. A post-processing step – such as applying a protective coating at the outer surface – may be part of the manufacturing processing. And for products that consist of several materials, joining steps are included.
Next part of the manufacturing – structure – properties – application chain is the whole of material properties. Different materials have different properties, which characterise them and make these materials distinctive. A property indicates how a material responds to external factors. For example, a material can bend or break – or apparently ‘do nothing’ – when you subject it to a sufficiently large mechanical load. A material can conduct an electrical current, or refrain electrons from flowing through it. A material can absorb, transmit, reflect or even emit light, or scatter light internally. A material can conduct or retain heat to a high or low extent, or may be able to resist high temperatures. This overview (see figure 3) compares the three main groups when looking at several properties.
Figure 3: Properties of metals (green), ceramics (blue) and polymers (red) in a nutshell
Let’s take a few concrete examples to clarify parts of the structure – properties – application chain. How does the microstructure determine the properties of a material, and the applications? How do several materials behave when they get into contact with heat, light, electricity and mechanical loads?
Light or heavy?
Why are some materials light, and others heavy? When you want to compare the weight of materials with each other, it is best to compare their density, which is the mass per volume unit. The light/heavy distinction is mainly due to the mass of atoms of which the materials are composed, and much less due to their volume. Polymers consist mainly of light carbon, hydrogen and oxygen atoms. Metals consist of relatively heavy metal atoms. Ceramics are usually compounds consisting of a heavy metal atom and a light non-metal atom, and their density is in between those of polymers and metals. If you think that a brick is heavy: a gold bar of the same size weighs almost ten times as much! (brick density ~ 2 kg/liter, gold ~ 20 kg/liter). This light/heavy distinction between the main groups polymers, metals and ceramics is not absolute: after all, aluminium and magnesium are called ‘light metals’. Lightweight materials are especially important in dynamic applications – aluminium for bicycles, cars or aircraft – while heavy materials are mostly found in stationary applications. You can modify the density by mixing different materials (such as in composites), or by mixing with ’empty space’, so by introducing porosity.
Free electrons in metals
The important role of free electrons in metals is another example of how microstructure affects materials properties. Metals are known to be good electrical conductors – copper live wires are well known – because some electrons are so weakly attached to the atoms that they can easily be released, and can flow through the metal as a ‘sea of electrons’. They can easily flow from one place to another, and all these mobile carriers together form the electrical current.
However … these free electrons do even more: they conduct heat! Metals are good electrical conductors, and good heat conductors as well due to these free electrons. When the temperature increases, the electrical and heat conductivity decreases. What is happening here? At higher temperatures, the metal atoms will vibrate more strongly around their lattice position, and then hinder the free electrons in their pathway, causing them to move slower.
Metals reflect light; just think of a mirror – a thin layer of aluminium at the backside of a glass pane. If light hits the metal surface, the many free electrons absorb the energy of the light in a fraction of a second. But such an electron does not feel comfortable at this high energy level, and would like to return to its original state – releasing the energy of the captured light. The incident light is reflected, and this explains the mirroring surface of metals. The other way around: in general, you can say that a material allows light to pass if the material and the incident light do not interfere. Optical permeability and electrical conductivity usually do not go along together.
The role of free electrons in metals is even bigger. Because these electrons are no longer bound to one position, the remaining rows of metal ions can easily slide along each other. And this is the basis for the plastic deformation of metals when a large force is applied. They do not break immediately, but deform first, and absorb a lot of energy during this process.
Material imperfections on an atomic scale, better known as defects, form the basis for many material properties. Sometimes these defects have been introduced deliberately, as is the case in oxygen sensors or high temperature solid oxide fuel cells (SOFC). In both devices, the ceramic material zirconia is key. Its lattice structure contains oxygen vacancies, i.e. oxygen sites that are empty on purpose. If the temperature is high enough, adjacent oxygen atoms (in fact: ions) can jump to these empty sites, leaving other empty sites behind – where another oxygen ion can jump to.
In figure 4 you see a computer simulation where oxygen (indicated in red) moves to other ‘T positions’ of the lattice. Each red dot is an oxygen position recorded in time. In this way, transport of oxygen (ions) through the lattice occurs. In an oxygen sensor – such as the lambda sensor in your car that measures the oxygen content in the exhaust gases and returns this information to the engine to optimise combustion – the transport rate of oxygen through the zirconia is a measure for the oxygen content in the exhaust gases.
Figure 4: Diffusion via point defects of an oxygen ion conductor
Something similar holds for computer chips, which consist essentially of silicon. Silicon in its pure form is an electrical insulator. All blue electrons that you see in figure 5 are stuck in the bond between a silicon atom and its neighbours, and can not move freely through the lattice. But if you add a little phosphorus (P) as can be seen in this figure, the material is transformed into a semiconductor – between insulator and ‘real’ conductor. This phosphorus has an extra electron – shown here in red – that is not part of the bonds, and that can jump through the material, resulting in electrical conductivity to some extent.
Figure 5: Intentionally induced defects – silicon as a semiconductor
Previously, we already mentioned the rows of metal ions that slide easily alongside each other, thus forming the basis for the mechanical behaviour of metals by means of plastic deformation. If metals were perfect materials, they would be 10 to 100 times stronger than they really are. But metals are not perfect – not even pure metals. They contain defects that are called dislocations, and they make those metals much weaker than their theoretical strength. These defects are the edges of the half lattice planes in figure 6. These half lattice planes have entered into the lattice by ‘accidents’ during the crystal lattice growth, or by mechanical stresses in the lattice. Motion of dislocations causes plastic deformation of metals and results in low strengths. When you would – in your thoughts – push against the half lattice plane, it slides in the direction of the pushing action, although the atoms remain in position. Deformation that costs relatively little energy.
Figure 6: (Un)intentionally induced defects – dislocations in metals
Transparency depends on microstructure
Figure 7 shows an example of optical properties of a material, and how it relates to its microstructure. The ceramic material alumina is completely transparent, translucent or opaque – depending on the structure, and therefore on the manufacturing process. Light scattering is the keyword here.
We go from the right to the left in this figure. Alumina, as an example of advanced ceramics, is manufactured by pressing powder particles and baking them together (‘sintering’) in order to provide a strong body. Most alumina products do not reach a 100% density after sintering. Boundaries between the original powder grains and remaining pores between the grains – both in the same order of magnitude as the wavelength of visible light – are sources for light scattering inside these products, making them opaque.
If you sinter the alumina even further – and use magnesia as a sintering aid – then almost all pores between the grains disappear. There are still some grain boundaries between the alumina grains – see the figure in the middle – which allow for some scattering, which means that the material is not ‘transparent’ but ‘translucent’. This material is used in the well-known orange high-pressure sodium lamps along the highway. Alumina is translucent and resistant enough to withstand the corrosive properties of sodium and the high pressure involved.
The left-hand figure shows single-crystal alumina – also known as sapphire – which is one pure crystal without sources of scattering as grain boundaries or pores. Such a crystal is manufactured by taking a very small seed crystal and immersing it into molten alumina. Carefully pull up this seed crystal, and the attached molten alumina cools down and crystallises into the same structure as the seed crystal. Sapphire is used as camera lens in smartphones.
Figure 7: Several kinds of transparency for alumina
How does polystyrene foam insulate?
From light we move on to heat. Polystyrene foam is a material that is used as a heat insulator – and thus conducts heat very poorly. How does this occur? Heat can be transported by means of convection (think of warm air above a radiator), radiation (such as solar radiation) or conduction (as in heat exchangers in industry). For domestic applications such as home insulation, the ambient temperature is too low for heat radiation to play a role. Furthermore, the structure of polystyrene foam is such that convection and conduction hardly occur: due to the isolated cavities in the foam, gas flow can not occur, and heat conducts poorly through the low conducting gas/air that is present inside the cavities.
Now we have seen a few examples of how materials behave in the presence of electrical stimuli, light and heat. But what about mechanical stress, if you pull or bend a material?
Within materials science, a ‘stress-strain curve’ is widely used, and in figure 8 you can see a representation for a typical metal. A test piece of material is mechanically loaded by pulling it (‘stressed’, shown on the vertical axis), and on the horizontal axis you see the elongation (‘strain’) that results from it. Such a stress-strain curve consists for most materials of an elastic regime (where ‘stress’ is proportional to ‘strain’) and a plastic regime. In the elastic regime, if you remove the load, the material returns to its original state, hence the name ‘elastic’. The end of this elastic regime is called the ‘yield strength’. If you load the test piece even further, you will enter the plastic regime where the material is subject to permanent deformation, and if the load is large enough, the material may eventually break.
Figure 8: Mechanical properties: stress-strain curve for a metal
As a user of a material you are especially interested in the elastic regime; the yield strength determines the maximum allowable load, and you’d rather like it to be as high as possible.
However, as a manufacturer of a material, the area above the yield strength is quite interesting, because there you can use plastic deformation phenomena to make the material even stronger. By playing around in the area between the ‘practical strength’ (the yield strength) and the ‘ultimate strength’ (the tensile strength) you can further strengthen a material. Here we take again metal as an example.
As mentioned before, the motion of dislocations determines the strength of a metal. How can you make a metal stronger? Quite simple: make it harder for the dislocations to move by hindering them, as shown in figure 9.
– For example by manufacturing a metal alloy by incorporating foreign atoms into the metal. Bronze and brass are alloys of copper, with tin and zinc as foreign atoms.
– Or by incorporating large foreign particles in the metal, for example aluminium with ceramic silicon carbide particles, or super alloys in gas turbine applications.
– Or by manufacturing materials with small grains, which means that there are many grain boundaries that hinder the motion of dislocations. Small grains therefore provide stronger materials.
– Or by loading the material deliberately above its yield strength, which generates additional dislocations due to plastic deformation that interfere with each other (work hardening). Think of a blacksmith that hits a red-hot piece of metal with a sledgehammer.
Figure 9: Strengthening of metals by hindering of dislocation motion
Smart piezoelectric materials
Finally a type of material that is in fact a system in its own: piezoelectric materials, where electrical and mechanical properties are interconnected. The material has the capability to change shape – for example become shorter or wider – by applying an electric voltage over it. Or the other way around: compressing or otherwise deforming the material generates an electric voltage. A gas igniter is often based on piezoelectric materials, as well as the parking sensor of your car. You will find the material also in ultrasound imaging.
When looking at the microstructure of a piezoelectric material – in this case the ceramic material lead zirconate titanate or PZT in brief – you will understand the way it works. Figure 10 shows a piece of its crystal structure. Although the entire unit cell itself is electrically neutral – equal amounts of positive and negative charges – there is a charge distribution within this cell, because the positively charged ‘yellow’ ion isn’t located exactly in the cell centre but slightly above this centre. If you apply an electric voltage over such a cell (upwards, for example) then you push the positive ions in the direction of that field, and you pull the negative ions in the opposite direction. This way the distance between the oppositely charged ions changes, which changes the shape of the unit cell. So if you have a lot of these cells next to or above each other, the entire material will be deformed on a macroscopic scale.
Figure 10: Unit cell of the piezoelectric material PZT
Eddy Brinkman writes technical background stories in the fields of chemistry, materials science and information technology. On Wednesday May 31, 2017 he gave a presentation on ‘Materials science in a nutshell’ during the Materials 2017 trade fair – in fact a summary of his book ‘Kennismaken met materialen’ (ISBN 978-90-79926-00-8, 192 pages, Dutch language) that has been published in 2016. You can order this Dutch book by sending an e-mail message to firstname.lastname@example.org.
© 2017 Eddy Brinkman / Betase BV